Mobile aerial towers or lifts conventionally comprise a pair of elongated booms which are articulated or pivotally joined together. The lower end of one of the members is pivotally mounted upon a mobile platform while the opposite end of the other boom pivotally carries a bucket in which the operator rides. The platform is supported for rotation about a vertical axis to thereby provide for lateral swinging movement of the bucket, and the booms pivot about horizontal pivot axes to facilitate vertical movement of the bucket as well as fore and aft movement of the bucket. Three hydraulic or other suitable motors are provided for effecting three different movements of the articulated booms. One hydraulic motor is operative to control rotary motion of the platform about the vertical axis of rotation. Another of the hydraulic motors comprises a cylinder operative to swing the lower boom about its pivotal connection with the platform. The third hydraulic motor comprises a hydraulic cylinder which functions to cause pivotal movement of the upper boom with respect to the lower boom.
Hydraulic control valves are used to control the operation of the three hydraulic motors to thereby effect movement of the booms. The prior art structures have also included a hydraulic control system mounted with the bucket and connected to the control valves to permit the operator in the bucket to control operation of the boom. A preferred control mechanism is illustrated in the Myers U.S. Pat. No. 3,133,471 issued May 19, 1964. The control mechanism of that patent provides a plurality of hydraulic control valves operably connected by a plurality of pairs of hydraulic lines extending along the length of the boom to the valves controlling the hydraulic motors. One of the advantages of the arrangement provided by the Myers patent is that it permits the operator to precisely control movement of the articulated booms. Movement of the booms either horizontally or vertically tends to cause the operator's weight to be shifted. The control arrangement of Myers prevents feedback by providing controlled movement of the control handle such that the operator's momentum during movement of the bucket does not cause the operator to move the control handle too far thereby causing overreaction or overtravel of the bucket.
One of the disadvantages or drawbacks of the prior art constructions is that each hydraulic control function of the control valve requires a pair of hydraulic control lines extending the full length of the booms and connected to the valves. Accordingly, it is common to have at least six hydraulic lines extending the length of the boom.
Another feature of the prior art control arrangements is that the functions which can be accomplished by the control arrangement at the bucket are limited by the complexity of those systems having a pair of hydraulic fluid lines extending the full length of the boom for each control function. Other arrangements have a captive air system for each function, such air systems being very cumbersome and inaccurate due to the inherent compressive nature of air.